As shown in FIG. 1, the conventional six-diode travelling wave sampler, for example as used in the sampling head of a digital oscilloscope, comprises two chains of three series-connected diodes 2, 4, 6 and 8, 10, 12, the two chains being connected in parallel with opposite polarity. Short delay lines 14 and 16 are connected between the diodes 4 and 6 and the diodes 10 and 12 respectively. The diodes are biased into the off state by diode chains 20 and 22 and bias voltage sources 28 and 30. In order to sample the signal present at the input terminal 24, negative-going and positive-going strobe pulses are applied to the cathode of the diode 6 and to the anode of the diode 12 by a strobe generator 18 by way of coupling capacitors 32 and 34 and transmission lines 36 and 38. The diodes 2-12 are turned on at the leading edge of the strobe, and therefore charge is permitted to propagate from the signal input terminal 24 to the delay lines 14 and 16 by way of the diodes 2, 4, 8 and 10 and transmission lines 40 and 42. Charge continues to propagate through the diodes 2, 4, 8 and 10 and the transmission lines 40 and 42 to the delay lines 14 and 16 so long as the diodes 2-12 remain on. The diodes are turned off at the trailing edge of the strobe, and therefore signal charge present on the delay lines is trapped between the diodes 4 and 6 and the diodes 10 and 12. The rise time of the sampler (the time for which signal charge is permitted to accumulate on the delay lines) is approximately equal to the round trip time on the delay lines 14 and 16, e.g., 25 ps. The charge that is trapped on the delay lines has a common mode component that is related to the voltage of the input signal. The common mode charge is applied to a summing node A at the input of an error amplifier 26. An inverting amplifier 27 has its input connected to the input terminal 24 through a resistor 46. The charge received by the amplifier 27 is inverted and injected into the summing node A through a variable capacitor 48. The output terminal of the amplifier 26 is connected to apply positive feedback to the bias network for the diodes 2-12 through an integrator 50 having a memory gate. The memory gate receives a memory gate pulse from the strobe generator, and the memory gate is conductive during the memory gate pulse. The leading edge of the memory gate pulse is synchronous with the strobe pulse. At the end of the memory gate pulse, the memory gate becomes non-conductive and breaks the positive feedback loop, in order to prevent uncontrolled oscillation. The positive feedback provided by the amplifier 26 adjusts the level at which bias is applied to the diodes 2-12, so that immediately following each strobe pulse the amplifier 26 provides an output voltage that represents the difference between the input signal voltage at a sampling point and the input signal voltage at the previous sampling point.
One of the problems that arises in a diode sampler is known as blowby distortion. Blowby is the transfer of charge through a diode when in the off state. In a travelling wave sampler, blowby affects the quantity of charge that is captured on the delay lines when the diodes are in the on state and therefore distorts the value of the error signal that is generated by the error amplifier and is fed back to the bias network. In the FIG. 1 sampler, blowby is compensated by the resistor 46 and capacitor 48, which transfer to the input of the amplifier 26 a charge that is opposite in polarity but substantially equal in magnitude to that provided to the amplifier 26 through the diodes 2, 4, 8 and 10, the resistors 52 and 54 and the coupling capacitor 56 when the diodes are in the off state and therefore cancels the effect of the charge transferred through the diodes. The series combination of resistance and capacitance in each circuit branch leading from the terminal 24 to the amplifier 26 acts as a high-pass filter, blocking low frequency components in the charge transferred to the amplifier. The diodes 2 and 8 are connected in series with the diodes 4 and 10 in order to reduce the capacitance of each branch of the sampler and thereby increase the cut-off frequency of the high-pass filters. Although the capacitor 48 is variable, allowing manual adjustment to match the magnitude of the charge transmitted through the capacitor 48 to that transmitted through the capacitance of the diodes 2 and 8, this method of compensating blowby is subject to the disadvantage that it cannot readily be adjusted, in particular during operation of the sampler.
A second limitation to which the FIG. 1 sampler is subject arises from the fact that the strobe pulses must be of sufficient amplitude that they are able to overcome the sum of all the reverse biases of the diodes 2-12 in order to turn on these diodes at the leading edge of the strobe and to turn off the diodes at the trailing edge of the strobe and thereby trap the signal charge on the delay lines. The reverse bias of the diodes 2 and 8 must be sufficient that these diodes do not turn on between strobe pulses. Therefore the amplitude of the strobe pulses imposes a limit on the dynamic range of input signals that can be sampled accurately.